It became known as the 'North Cork Triangle' because the two cases had followed a remarkably similar pattern.

Conor and Sheila Dwyer, a couple in their early 60s who had followed a 'clockwork routine', disappeared without a trace from their home at Chapel Hill near Fermoy.

The last time they had been seen was on April 30, 1991.

Eleven months before, Labour Party councillor William Fennessy (53) had vanished without a trace outside Fermoy, on March 30, 1990.

In both cases, the car had also disappeared. For the Dwyers, it was their white Toyota Cressida - a luxury, powerful sedan built on slim lines - while Mr Fennessy's vehicle was a Daihatsu Charade.

For years, the two cases were tied together in the minds of the gardaí and the public, twin mysteries. Retired garda Joe Watkins, in a subsequent interview, said it had reminded him of 'Star Trek' - "Beam me up, Scotty, something like that," he said.

But then came an accidental breakthrough - when members of the Blackwater Sub Aqua Search and Rescue team stumbled upon the wreckage of a car on the bed of the River Blackwater in 2013.

The discovery had been made during a routine dive.

The decaying vehicle was about 3.5m below the surface and buried in silt.

DNA testing on the remains inside confirmed they were those of William Fennessy.

This ended talk of a 'North Cork Triangle'.

But the Dwyers remain missing - and this week, the gardaí said tracing the car, which bore the registration 5797 ZT, is the key to solving this mysterious case.

Newspaper archives reveal how on March 3 1992, a year after their disappearance, Garda divers searched the waters around Cobh in a bid to solve the riddle.

"They seem to have disappeared off the face of the earth," an article said.

It revealed that the possibility of a suicide pact, on which some friends had speculated, had been investigated, but no evidence had been produced to support such a theory.

And the search of the quays around Cobh had been "fruitless", the piece added.

Conor Dwyer (62) was described as being 5ft 8in in height, weighing 11 stone and with dark hair parted to the right.

When last seen, he was wearing dark rimmed glasses, a checked sports jacket and a light navy trench coat.

Press reports described him as a "flamboyant character" who had worked for a number of years as a chauffeur for Fritz Wolff, a German millionaire who holidayed at nearby Castlelyons House.

Sheila (60) was 5ft 4in with fair hair, and was wearing a light woollen coat and white leather walking shoes when she disappeared.

Mrs Dwyer had a telephone conversation with her two sisters, Maisie and Nellie, on May 1, 1991, and that was the last time there was any contact.

The evening before, the couple had been at a funeral and had greeted a neighbour's child on the steps of the church.

They were not reported missing until three weeks later.

The gardaí went to the Dwyer's neat terraced house and found the property secure, with all personal items such as clothes, passports and money still in the house.

A biscuit tin containing a thousand pounds was the only unusual thing they discovered.

In an RTE documentary some years ago, those few neighbours in Fermoy who still remembered them recalled them as "nice people".

"They were almost like royalty, with a big posh car. She used to wave like the queen," one woman said.

Their son, Conor, who lives in the UK, as does their other son, told RTÉ that his childhood had been a happy one.

"Good neighbours, good friends, well respected normal people," he said, adding that it was strange that while they were normal people, there had definitely been something abnormal about what had happened to them.

"I can't see any rhyme nor reason for the entire situation," he said.

"They were local, friendly, respected people.

"There are lots of questions, unanswered questions that nobody seems to know the answers to.

"Unfortunately the only answer I had was 'I don't know'," he said.

At that time, he said, he often wondered about what was going through his parents' minds and what happened.